The invention relates to a device for inspecting parts by eddy currents, the device comprising at least one measurement bridge having an active coil which is connected between a first terminal and a second terminal, and a reference circuit which is connected between said second terminal and a third terminal, said first and third terminals being for connection to an AC voltage generator so that an alternating magnetic field is generated by the active coil in a part to be inspected in order to enable any variation in the impedance of the active coil on passing over a defect in a part under inspection to be detected by analyzing an output voltage of the device.
Non-destructive inspection by means of eddy currents is in widespread use in industry for verifying the mechanical state of metal parts, since such testing enables irregularities such as break starters and surface state defects in such parts to be detected. By using an active coil to give rise to an alternating magnetic field in a metal part, currents are caused to flow in said part which modify the magnetic field induced by the coil, and this modification to the magnetic field is reflected by a modification in the impedance of the active coil.
By way of example, in the steam generators of nuclear power stations, the pipework which operates under severe conditions of temperature and pressure needs to be inspected regularly. Such generators typically comprise a support plate having a multitude of U-shaped tubes inserted therein, with heat exchange taking place via the tubes. Such tubes are suitable for being inspected by eddy currents. Their accessibility means that it is possible to insert a probe containing an active coil, and to move the coil while it is generating an alternating magnetic field. Simultaneously, by analyzing an output voltage, it is possible to detect any variation in the impedance of the active coil as it passes over a defect in the part. In such inspection, it is particularly advantageous to perform measurements of a given portion of tube at a plurality of frequencies in order to evaluate a magnitude concerning the size of the defect whose shape is to be discovered. This type of measurement is particularly useful for inspecting tube portions that are inserted in a support plate since they enable the influence of the plate to be distinguished from the influence of the tube itself.
Depending on circumstances, conventional inspection devices comprise either a single bridge for performing measurements in a so-called xe2x80x9cabsolutexe2x80x9d mode, or a plurality of bridges for performing measurements both in differential mode and in absolute mode. In general, a measurement bridge comprises a first terminal connected to the active coil, referred to as the positive injection terminal; a second terminal connected to the active coil and to the reference circuit, referred to as the output terminal; and a third terminal connected to the reference circuit and referred to as the negative injection terminal.
For an inspection device comprising a single bridge, and thus supplying a single output voltage, the active coil placed in a probe body is connected in series with a reference circuit so as to constituted a measurement bridge. The bridge constituted in this way is powered between its positive and negative injection terminals by an AC voltage generator, and the output voltage from the device is the voltage read between the output terminal and the negative injection terminal, which is usually connected to ground. In such a device, the reference circuit conventionally used is a resistor whose impedance at the working frequency is close to that of the active coil. Thus, the form of the output voltage is close to half the power supply voltage when the bridge is in balance, and it departs significantly therefrom when the bridge is out of balance. To perform inspection in absolute mode, the probe is moved inside the tube to be inspected, and the output voltage which has a certain form (amplitude and phase shift) for a good portion of tube departs from said form when the active coil passes over a defect.
In terms of orders of magnitude, it should be observed that the output voltage obtained has a mean value which is of the order of a few volts, whereas the variations which appear are of the order of a few millivolts. It is therefore essential to have signal processing means to filter the output voltage and to amplify those components therein that are meaningful so as to show up any variation in the impedance of the coil. To do this, use is made of a synchronous demodulation that is capable of operating in real time and of supplying data which a computer records, likewise in real time.
U.S. Pat. No. 4,635,832 describes a method and apparatus for casting molten metal covered in a layer of slag from a ladle into a tundish. The bottom of the ladle is connected to the tundish via a tube through which the molten metal flows. That document describes a measuring circuit for detecting the passage of slag along the tube connecting the ladle to the tundish: the tube is surrounded by two coils that are connected in series and that extend one after the other along the tube. The coils are fed from the secondary of a transformer that itself comprises two coils in series, forming a Wheastone bridge.
The measurement diagonal is taken between the point interconnecting the two coils surrounding the tube and the point interconnecting the two coils of the secondary of the transformer. If any slag passes along the tube connecting the ladle to the tundish, then the output signal is observed to vary due to the coil which is affected by said passage having its impedance modified. Nevertheless, by mutual inductance between the two coils, the other coil is also affected and therefore cannot constitute a reference coil.
The present invention proposes a device in which, in the absence of any fault, the output signal is close to zero and in which the reference coil is protected from the influences of the part under inspection.
The invention thus provides a device for inspecting a part by eddy currents, the device comprising at least one measurement bridge having an active coil which is connected between a first terminal and a second terminal, and a reference circuit which is connected between said second terminal and a third terminal, said first and third terminals being for connection to an AC voltage generator so that an alternating magnetic field is generated by the active coil in the part to be inspected in order to enable any variation in the impedance of the active coil on passing over a defect in a part under inspection to be detected by analyzing an output voltage of the device, wherein the reference circuit is a reference coil identical to the active coil, wherein the first terminal and the third terminal are for receiving voltages applied by the AC voltage generator which are equal in value and of opposite signs relative to a neutral phase of said generator, the output voltage being the voltage read between said second terminal and said neutral phase, and wherein the reference coil is electromagnetically isolated from the part to be inspected.
According to another characteristic, the device comprises a plurality of measurement bridges having common respective first terminals, common respective third terminals, and different second terminals, thereby obtaining an output voltage for each measurement bridge which is read between the second terminal of the corresponding bridge and the neutral phase of the AC voltage generator.
In a first embodiment, the active coil(s) is/are situated in a probe body, the reference coil(s) being situated outside the probe body and being remote from the part under inspection.
In a second embodiment, the active coil(s) and the reference coil(s) are situated in a single probe body, the reference coils being isolated by electromagnetic shielding.
According to another characteristic, the second terminal of each measurement bridge is connected to at least one operational amplifier.
With this arrangement, the output voltage is very close to zero if the bridge is in balance since it is close to the sum of the injection voltages which are opposite at a given instant. Thus, the output voltage that is obtained does not need any special filtering so it can be amplified directly prior to demodulation. With such a device, it is possible to obtain a raw signal-to-noise ratio of about 40 dB, which represents a significant improvement over the 25 dB provided by a conventional device.
Another advantage of the device of the invention is that it is capable of operating at a plurality of frequencies without any need to change the reference circuit. This characteristic is particularly advantageous since units are available on the market comprising a voltage generator and a synchronous demodulator that are capable of operating in real time at a plurality of frequencies: with such a unit, it is possible to inject an AC voltage into the device that is made up of a plurality of voltages at different frequencies, and to demodulate each output voltage as a plurality of voltages each corresponding to one of the injected frequencies. As can be seen, associating such a unit with the device of the invention makes it possible to perform measurements at a plurality of different frequencies on a single passage of the probe in the part.